Law enforcement officials said that Adam Lanza was armed with four firearms when he started his rampage at a Connecticut elementary school Friday that ended in the deaths of 20 children and seven adults, but nearly all the killing was done with just one of the guns: a .223 caliber Bushmaster semiautomatic rifle.

Lanza used the rifle, a modified civilian version of the military's M-16 similar to the popular AR-15, as he stalked through the school and opened fire on children as young as five. Dr. H. Wayne Carver, the medical examiner who investigated the massacre, told reporters over the weekend that all of Lanza's victims had been shot more than once.

The killing ended when Lanza took his own life, this time using a handgun, officials said.

Bushmaster, headquartered in North Carolina, bills itself on its website as the leading supplier of AR-15-type rifles in the U.S. and offers more than a dozen different models in various calibers.

A February report by Guns and Ammo magazine noted a growing demand in recent years for AR-15-type rifles – and specifically those loaded with .223 caliber bullets – for use in home defense. The .223 caliber load is popular, the article says, because it has better fragmentation upon impact, meaning it will deal a lot of damage with less chance of accidentally continuing through the target and endangering whoever's in the background.

The magazine reported that 1.5 million AR-15s were made in the last five years alone – one for every 209 Americans.

"This thing is just a killing machine," Josh Horwitz, Executive Director of the Coalition to Stop Gun Violence, told ABC News today. "It's designed, like I said it was designed... very similar to the weapon that's used in the battlefield."

Sources told ABC News today that the guns used in the massacre, including the Bushmaster, were purchased by Lanza's mother, Nancy. She bought the Bushmaster in March 2010, a Sig Sauer handgun the next year and then a Glock handgun in January 2012.

Authorities say that Nancy and Adam Lanza went to the firing range together in the last six months. The director of one local firing range told ABC News that ATF agents had looked for the Lanzas' names in his firing logs for 2012, but the names did not appear.

"Unfortunately from his mother's standpoint, she thought it was a way to bond with him," said Brad Garrett, an ABC News consultant and former FBI profiler. "It's a terrible idea. You have a child that's obviously troubled, doesn't bond with people, doesn't want to bond with her, apparently, and then you introduce something that's so lethal and so violent, inappropriately used. It's just an extremely poor decision on her part."

Connecticut's gun laws are some of the toughest in the country, according to anti-gun groups, but they do not specifically ban the Bushmaster AR-15-type guns and the weapon can be easily modified to dodge other restrictions. On Bushmaster's website, the company offers to help customers make sure their assault-style rifles are "state compliant."

"But it's still just as deadly because what makes it dangerous is the ability to take almost unlimited amounts of ammunition and a pistol grip," said Horwitz. "That's what allows the shooter to keep the barrel down on the target."

On its website, the gun company says that "With a Bushmaster for security and home defense, you can sleep tight knowing that your loved ones are protected. Bushmaster offers everything you need to ensure the safety of you and your family. ... And with their intimidating looks, all Bushmasters make a serious impression. Any gun will make an intruder think. A Bushmaster will make them think twice."

Scott Ostrosky, a local shooting range owner in Connecticut, told ABC News that many of his customers are attracted to the gun because of its similarities to the military weapons, but they're generally responsible people who are just target shooting for fun.

"Yeah, it's fun," he said. "We shoot glass bottles. We shoot up old beer kegs, just garbage items. We get a kick out of shooting... Some people fancy high-dollar golf clubs for their chosen recreation, other people fancy a gun like that for their recreation."

But Brad Garrett said he thought the Bushmaster had "empowered" Lanza. "And once he had decided at some point he was going to commit this mass shooting," said Garrett, "he now has been trained with a piece of equipment that will accomplish that."

Lanza was not the first to take a Bushmaster outside the realm of home defense and recreation and into the bloody pages of a national tragedy.

In 2002 the men known as the Beltway snipers, John Muhammed and Lee Boyd Malvo, used a Bushmaster .223 to shoot more than a dozen people. In that case, Bushmaster contributed $500,000 to a multi-million-dollar settlement with the victims' families, who claimed the gun manufacturer didn't do as much as it should to keep the guns out of the hands of criminals. Bushmaster said its contribution to the settlement did not amount to an admission of guilt.

Representatives for Bushmaster did not respond to an emailed request for comment for this report.